The present invention relates to an apparatus for printing a color superposition image, and particularly relates to a roller offset printing apparatus adapted to production of patterns such as liquid crystal color filter patterns, exposure mask patterns, liquid crystal shadow mask patterns, etc.
Referring to FIGS. 6 and 7, there is shown a conventional roller offset printing apparatus which has a roller transfer drum 101 covered with a rubber sheet and adapted to be rotated by a rotation motor 102, a linear stage 103, pinions 101a and 101b provided at opposite ends of the roller transfer drum 101, racks 103a and 103b provided at opposite side edges of the linear stage 103 so as to be engaged with the pinions 101a and 101b respectively, a master plate 106 and a work plate 107 disposed at a predetermined interval on the linear stage 103, and ink pits 121 (the diagrams (b) and (c) of FIG. 7 for reserving ink in a surface of the master plate 106.
As shown in the diagram (a) of FIG. 7, the pinions 101a and 101b are engaged with the racks 103a and 103b respectively, so that the roller transfer drum 101 moves while rotating on the linear stage 103 when the rotation motor 102 is driven. When the roller transfer drum 101 rotates on the surface of the master plate 106, ink 120 in the ink pits 121 is transferred onto the outer circumferential surface of the roller transfer drum 101 (the diagram (b) of FIG. 7). When the roller transfer drum 101 further rotates on the surface of the work plate 107, the ink 120 is transferred from the outer circumferential surface of the roller transfer drum 101 onto the surface of the work plate 107 (the diagram (c) of FIG. 7).
Thus, the conventional apparatus uses such a driving system that the roller transfer drum 101 is rotated through the engagement between the pinions 101a and 101b and the racks 103a and 103b as described above. Accordingly, a displacement may be produced between an original image on the master plate 106 and a printed image on the work plate 107 because of backlash, abrasion, etc. This arouses a problem in superposition printing.
Further, in the conventional apparatus, slipping of the roller transfer drum 101 occurs on the surface of the master plate 16 or on the surface of the work plate 107 so that high accurate printing is prevented.
Further, referring to FIGS. 11 through 13, there is shown another conventional known roller offset printing apparatus which has a roller transfer drum 130 adapted to be rotated by a motor not shown, a stage 133 provided with a master plate 131 and a work plate 132 placed thereon, left and right bearings 134 for supporting opposite ends of the roller transfer drum 130, left and right bearing accommodation boxes 135 for storing the bearings 134 respectively, left and right stands 136 for supporting the respective boxes 135 so that the respective boxes 135 can move up and down, left and right cylinders 137 for moving up and down the respective boxes 135, a rubber sheet (blanket) 138 wound on the outer circumferential surface of an iron roller 130a of the roller transfer drum 130, left and right pinions 130b formed at the opposite sides of the roller transfer drum 130, left and right bearers 130c formed at the opposite side edges of the roller transfer drum 130, left and right racks 133a provided at opposite sides of the stage 133 so as to be engaged with the pinions 130b respectively, and rails 133b provided at the opposite side edges of the stage 133 and having upper surfaces equal in level to the surfaces of the master and work plates 131 and 132.
In the conventional apparatus, the external size of the rubber sheet 138 is set so as to be equal to the external size of the left and right bearers 130c. Accordingly, when the roller transfer drum 130 is moved by the left and right cylinders 137 down to a position where the left and right bearers 130c come into contact with upper surfaces of the left and right rails 133b, the outer circumferential surface of the rubber sheet 138 comes into contact with the surface of the master plate 131 or the surface of the work plate 132 as shown in the diagram (a) of FIG. 13 and at the same time the left and right pinions 130b are engaged with the left and right pinions 133a as shown in FIG. 11. Further, in the conventional apparatus, a printing pressure adjustment shim 139 is disposed between the iron roller 130a and the rubber sheet 138 as shown in FIG. 12 in order to generate a predetermined printing pressure. Accordingly, when the transfer drum 130 is moved down as described above, the rubber sheet 138 is elastically deformed by the thickness .delta. of the shim 139 as show in the diagram (b) of FIG. 13 so that a predetermined printing pressure is generated.
Accordingly, in the conventional apparatus, when the printing pressure is to be changed, there is required the printing pressure adjusting work of removing the rubber sheet 138, replacing the shim 139 to a new one having a thickness in accordance with the printing pressure and then winding the rubber sheet 138 on the outer circumference of the new shim again. There arises a problem in that a long time is required for the printing pressure adjusting work because the printing pressure adjusting work is troublesome. There arises another problem in that a desired printing pressure cannot be set with high accuracy because of the variation of tolerance in the shim 139 and the error in the mounting of the shim 139.
In addition, in the conventional roller offset printing apparatus shown in FIG. 11, the rubber sheet (blanket) 138, as an elastic member, is wound on the outer circumferential surface of the iron roller 130a of the roller transfer drum 130. In the conventional apparatus, the ink in the ink pits (patterned recesses) in the surface of the master plate 131 is transferred onto the outer circumferential surface of the rubber sheet 130a and further transferred onto the surface of the work plate 132 when the roller transfer drum 130 is moved while being rotated on the surface of the master plate 131 or the surface of the work plate 132 placed on the stage. Thus, patterns on the master plate are printed on the work plate.
The rubber sheet 138 however expands and sags gradually because of the printing pressure (printing pressure) and the chemical reaction of the ink. When such sagging occurs, patterns on the master plate cannot be reproduced on the work plate accurately. There arises a problem in that printing accuracy deteriorates.
Further, in the conventional roller offset printing apparatus as shown in FIG. 11, opposite ends of the roller transfer drum 130 are supported by the left and right bearings 134 respectively. The left and right bearing accommodation boxes 135 for storing the bearings 134 are supported by the left and right stands 136 respectively so as to be movable up and down. Projections 135a are formed at opposite ends of each of the bearing accommodation boxes 135. The projections 135a, opposite end portions of each of the stands 136 and push plates 139 fixed to each of the stands 136 make up a linear guide.
In the conventional apparatus, the boxes 135 are supported by the stands 136 respectively so as to be movable up and down through the slide engagement of the projections 135a of the left and right boxes 135 with grooves formed by the left and right stands 136 and the push plates 139. Further, the engagement portions are provided with considerable gaps so that the boxes 135 can be moved up and down in a low friction. Accordingly, when the roller transfer drum (blanket drum) 1 is moved while being rotated on the surface of the master plate 131 and the surface of the work plate 132, the transfer drum 1 becomes unstable in the directions of the axis and radius thereof in accordance with the gaps. As a result, patterns on the master plate 131 cannot be reproduced on the work plate 132 accurately. There arises a problem in that printing accuracy deteriorates.